For a structure of a conventional planar circuit to waveguide transition, the following structure is proposed: a differential line constituted of a pair of signal line conductors provided on a dielectric substrate of a single layer is connected to a rectangular patch conductor to transit a signal to a waveguide disposed above the substrate by utilizing resonance of the patch conductor (for example, Non-Patent Document 1).
On the other hand, there is disclosed a transition including a coaxial mode to waveguide mode transition in an inner layer of a multilayer substrate to transit a differential mode to a waveguide mode (for example, Patent Document 1).